1. Field of the Invention
The present invention relates to a high zoom-ratio zoom lens system for a photographic camera, especially for a lens shutter camera.
2. Description of the Prior Art
Unlike a zoom lens system of a single lens reflex (SLR) camera which requires a space to accommodate a quick-return mirror behind the photographing lens system, a zoom lens system of a compact camera does not require a long back focal distance. As an example of such a high magnification-ratio zoom lens system of a lens shutter camera in which there is no substantial constrain on the back focal distance, a zoom lens system, including a lens group having a positive refractive power (hereinafter, a positive lens group), another positive lens group, and a lens group having a negative refractive power (hereinafter, a negative lens group), in this order from the object, has been proposed (e.g., U.S. Pat. No. 4,978,204). However, in such a zoom lens system, if an attempt is made to further increase the zoom ratio, the overall length of the zoom lens system inevitably becomes longer at the long focal length extremity.
Furthermore, for the purpose of achieving further miniaturization and a higher zoom ratio, a zoom lens system including a positive lens group, a negative lens group, a positive lens group and a negative lens group, in this order from the object, has been proposed (e.g., U.S. Pat. No. 6,002,529 and Japanese Unexamined Patent Publication No. 2000-180725). However, in such a lens arrangement, the traveling distances of the lens groups are longer, so that the overall length of the zoom lens system at the long focal length extremity becomes longer. Moreover, the frontmost lens diameter becomes larger since the entrance pupil position is distant at the short focal length extremity. Consequently, further miniaturization of the camera cannot be achieved.
The present invention provides a miniaturized and high-zoom-ratio zoom lens system in which (i) the overall length thereof can be made shorter at the long focal length extremity, (ii) the diameter of the frontmost lens group can be reduced, and (iii) appropriate optical performance is attained over the entire focal length regard less of a smaller number of lens elements.
More specifically, in the zoom lens system of the present invention, the zoom ratio Z (=fT/fW) is more than 5 (Z greater than 5); the overall length thereof TLT at the long focal length extremity is short to the extent that TLT/fT is less than 0.8 (TLT/fT less than 0.8); and the diameter of the frontmost lens group is small.
Accordingly, the present invention provides a high-zoom-ratio zoom lens system suitable for a lens shutter camera having in a retractable photographing lens system.
According to an aspect of the present invention, there is provided a high zoom-ratio zoom lens system including a first lens group having a positive refractive power (hereinafter, a positive first lens group), a second lens group having a negative refractive power (hereinafter, a negative second lens group), a third lens group having a positive refractive power (hereinafter, a positive third lens group), and a fourth lens group having a negative refractive power (hereinafter, a negative fourth lens group), in this order from the object.
Zooming is performed by moving each of the positive first through positive fourth lens groups along the optical axis.
The high zoom-ratio zoom lens system satisfies the following conditions:
0.2 less than LD23W/fW less than 0.70xe2x80x83xe2x80x83(1) 
0.02 less than xcex94D23/fW less than 0.2xe2x80x83xe2x80x83(2) 
11 less than |fT/f2| less than 14 (f2 less than 0)xe2x80x83xe2x80x83(3) 
12 less than fT/f3 less than 15xe2x80x83xe2x80x83(4) 
wherein
LD23W designates the distance from the most object-side surface of the negative second lens group to the most image-side surface of the positive third lens group, at the short focal length extremity;
xcex94D23 designates the difference in the distance between the negative second lens group and the positive third lens group at the short focal length extremity and the distance therebetween at the long focal length extremity;
f2 designates the focal length of the negative second lens group;
f3 designates the focal length of the positive third lens group;
fW designates the focal length of the entire the zoom lens system at the short focal length extremity; and
fT designates the focal length of the entire the zoom lens system at the long focal length extremity.
The high zoom-ratio zoom lens system preferably satisfies the following condition:
1.4 less than f23T/f23W less than 1.7xe2x80x83xe2x80x83(5) 
wherein
f23T designates the combined focal length of the negative second lens group and the positive third lens group at the long focal length extremity; and
f23W designates the combined focal length of the negative second lens group and the positive third lens group at the short focal length extremity.
The high zoom-ratio zoom lens system preferably satisfies the following condition:
0.05 less than |ra/fT| less than 0.15 (ra less than 0)xe2x80x83xe2x80x83(6) 
wherein
ra designates the radius of curvature of the most object-side surface of the negative second lens group; and
fT designates the focal length of the entire the zoom lens system at the long focal length extremity.
The positive third lens group includes at least two positive lens elements, and preferably satisfies the following conditions:
np less than 1.55xe2x80x83xe2x80x83(7) 
vp greater than 65xe2x80x83xe2x80x83(8) 
wherein
np designates the average refractive index, with respect to the d-line, of the positive lens elements in the positive third lens group; and
vp designates the average Abbe number, with respect to the d-line, of the positive lens elements in the positive third lens group.
Focusing is preferably carried out by integrally moving the negative second lens group and the positive third lens group.
The present disclosure relates to subject matter contained in Japanese Patent Application No. 2002-238868 (filed on Aug. 20, 2002) which is expressly incorporated herein in its entirety.